DIODE CIRCUITS 



From the point of view of regulation, then, there is an optimum value of C; 

 the calculations involved in finding it are complicated, but fortunately the 

 manufacturers of rectifying diodes are very helpful, in their pubHshed data, 

 with recommendations on this point. Practical values range between 4 and 

 32 /^F. 



rvn 



Figure 6.16 



It is now evident why the full-wave system of power rectification is so 

 much to be preferred. Diode conduction times occur twice as often {Figure 

 6.16) so for a given load and capacitance the peak diode current is halved, 

 thus regulation is improved. Alternatively, more capacitance can be used 

 for the same regulation, thus reducing the ripple. Another important point 

 is that the fundamental ripple frequency is doubled, which facilitates further 

 smoothing. 



1~ 



Peak to 



peak 



ripple 



Figure 6.17 



The ripple voltage may be estimated in the following way. Suppose all 

 the diode current per cycle flows instantaneously, then the output waveform 

 is triangular and the capacitance supphes load current for time T {Figure 6.17). 

 The rate of discharge for the capacitance is constant for all reasonable values 

 of C and equal to IjC volts per second where / is the load current. For half- 

 wave systems on 50 cycle mains, T = 0-02 seconds, so the peak to peak ripple 

 voltage is 0-02 IjC. For a full-wave system T — 0-01 seconds so the peak to 

 peak ripple is 0-01 //C 



•wWjIIIIJWa. 



out 



Figure 6.18 



Application to signal rectification {Figure 6.18) — For signal rectification 

 the load is usually constant and so regulation is not of any importance. 

 Further, power levels are low and there is no question of destroying the 

 diode. We are therefore free to vary C and see how doing this affects the 



102 



